The invention relates generally to the art of injection molding and more particularly to methods and apparatus for controlling motion and/or pressure in an injection molding machine.
Injection and other types of molding machines are complex systems, typically operated in multiple steps or phases, in order to provide a molded part or parts in a molding cycle. Once a finished part is removed from the machine, the molding cycle is repeated to produce further parts. A typical injection molding machine operational cycle includes clamp, inject, pack and hold, recovery, and eject steps, each of which involves moving machine components and motion control thereof. The clamp phase joins the individual sides or portions of a mold together for receipt therein of plastic molding material, in the form of a melt. In the inject phase, a reciprocating screw or ram within a cylindrical barrel pushes or injects a plasticized melt through an orifice at the barrel end or nozzle, which in turn provides the melt to the interior cavity of the mold. Further material is then provided to the mold and force is maintained during a pack and hold phase, and the eject phase separates the molded part from the separated mold halves. The screw is retracted in the barrel during a recovery phase while the screw is rotated to advance new plastic material through screw flights into the barrel space forward of the screw, whereupon the cycle may be repeated.
Each of the molding machine phases may involve linear and/or rotational motion of one or more machine components, which motions are implemented using appropriate actuators in a controlled fashion. The actuators for such machine component motion can be of various forms, such as hydraulic actuators, rotary and linear electric motors, and the like. The various rotational and/or translational motions of the molding machine components are often controlled according to predetermined profiles in order to achieve the desired molded parts, while attempting to ensure complete filling of the mold, reduce cosmetic and structural problems in the molded parts, eject molded parts without damage, and to minimize cycle time to achieve acceptable machine throughput. In addition to motion control, such molding machines also include controls and actuators for controlling temperatures, pressures, and other process variables in the machine. Many contemporary injection molding machines employ a combination of open-loop command type control for non-critical motions, such as for clamping and ejection, in combination with closed-loop control for injection ram velocity, hold pressure, back pressure, and screw rotation speed control.
Controls for injection molding machines have evolved from early manual controls wherein plastic was injected into a mold when a crank wheel was turned, to programmable logic controllers operating the machine actuators in closed loop fashion using sensor inputs to implement a control law, typically proportional, integral, derivative (PID) control. Conventional injection molding machine PID controllers receive sensor inputs indicative of machine component motion (e.g., position, velocity, etc.), which are then compared with desired motion values (e.g., set point values) to derive an error value. Control outputs are derived from the error value and are provided to actuators in the machine, such as hydraulic valves, electric motors, or the like, in order to reduce the error. PID controllers provide such control output signals which are proportional to the error, the integral of the error, and/or the derivative of the error, wherein PID coefficients kp, ki, and kd are set to provide relative weighting for the proportional, integral, and derivative components of the control output signal. More recently, molding machine controllers have provided more advanced functionality, such as combining auto-tuned PID with a predictive open-loop term and an adaptive learned disturbance correction term, as set forth in my U.S. Pat. No. 5,997,778, the disclosure of which is hereby incorporated by reference as if fully set forth herein. The present invention provides an improvement to the conventional molding machines and controls therefor illustrated and described in U.S. Pat. No. 5,997,778.
Motion of various components in a molding machine are commonly controlled according to a user-defined profile. For instance, a user may define a desired profile for ram velocity versus position or time to be used during an injection step, sometimes referred to as a velocity profile. Alternatively or in combination, such moving components may be controlled according to a user-defined pressure profile, such as melt pressure profile defined in terms of pressure with respect to time. A machine cycle, moreover, may employ one or more such profiles for various steps therein. For example, the injection ram translation may be controlled according to a velocity profile during the injection step, after which pressure profile type control is employed during a pack and hold step. Furthermore, different profiles and/or profile types may be used to control different subsystems within a molding machine. In this regard, velocity profiling may be used to control the ram during injection, whereas pressure profiling may be used to control a clamping system operation.
Where velocity profiling is used, the user defines the desired profile, which establishes the desired velocity of one or more moving machine components with respect to position. The controller then uses the profile as a series of set point values (e.g., and/or derives further set point values therefrom) for the control law in order to implement a particular machine phase in a molding cycle. For instance, the user typically defines a desired piecewise linear translational velocity profile with respect to position, for movement of the injection ram during an injection step. Alternatively, the user may define a pressure versus time profile for the melt in the barrel during injection, which can be used to control the linear ram force during injection. Similarly, user-defined profiles may be used in controlling rotational movement of the ram, heating of the barrel, clamping of the mold halves, movement of the carriage, and/or ejecting finished parts from the separated mold.
To facilitate user entry of such desired control profiles, molding machines often include an operator station or user interface with a screen display and keyboard which sends signals to a programmable logic controller (PLC). For example, such a user interface may allow an operator to define desired ram velocities at fixed ram travel increments or zones. The operator sets the desired ram velocity at each zone, from which a series of bar graphs are assembled. The ram movement is then controlled so as to follow the bar graphs. More recently, the bar graphs have been replaced with points at each zone boundary so that the ram is not programmed to travel at constant speed within each zone. Rather, the ram is controlled according to a linear interpolation which varies from one set point at one zone boundary to another set point at an adjacent zone boundary. Thus, in defining a number of desired velocity settings at set ram travel positions which the ram is to follow, a xe2x80x9cvelocity profilexe2x80x9d is established. The molding machine controller then attempts to cause the ram to travel at the user set speeds at the user set positions i.e., to emulate the velocity profile, by providing corresponding control output signals to one or more actuators in the machine.
In a conventional molding machine, the controller receives a velocity feedback signal from a velocity sensor on the machine and compares it with the user set velocity profile to generate an error compensated control output value by which the injection ram speed is controlled. Alternatively, a position signal is received from a ram longitudinal position sensor, and is differentiated, either in hardware or in software, in order to derive velocity information therefrom. Where the linear actuator for the injection ram is hydraulic, the control output value is converted to an analog control signal, which is then provided to a solenoid valve actuator regulating a hydraulic proportional flow control valve. The valve, in turn, controls flow from a pump to a prime mover causing ram movement. The user-defined velocity profile is typically converted into a series of velocity set points (e.g., and/or pressure set points) used by the controller in generating appropriate control output values during machine motion. The PLC thus provides closed loop control (e.g., via feedback signals from a ram position or velocity sensor) by providing a drive signal to the hydraulic control valve. Alternatively, where the ram is linearly actuated by an electric motor, the controller provides velocity signals as control outputs to a motor drive. The drive, in turn, provides corresponding current to the motor actuator in order to achieve the desired ram velocity. Such closed loop type control may also be combined with open loop control during certain non-critical portions of one or more motion control sub-systems.
Although PID and other type controls have thusfar been employed in controlling various motions in molding machines, improved control capabilities are desirable, in order to improve finished part quality, repeatability, and cycle time. For instance, non-linearities in the molding machine and high order energy storage prevent optimal control of the motion of various components, such as the injection ram. Such non-linearities include, for example, ball screw friction associated with the injection ram. In addition, energy storage occurs as hydraulic hoses in the system are subject to transverse expansion depending upon pressures in the hydraulic fluid. Electric motor type actuators also suffer from non-linear behavior, due to backlash, heating effects, noise, and the like. Furthermore, other disturbances in the machine cause non-linear or indeterminate behavior, such as hydraulic fluid leakage. PID and other controllers are of limited order and therefore cannot cancel high order dynamics of the system. Also, conventional controls require tuning of the various control coefficients (e.g., kp, ki, and kd) associated therewith, in order to achieve good control. However, tuning these coefficients may be beyond the capabilities of the end user of such molding machines. Moreover, the tuning capabilities provided by kp, ki, and kd in a PID controller are not sufficient to optimize the control operations of such machines, where non-linearities, high order dynamics, highly variable disturbances, and other indeterminate conditions are present. Consequently, there is a need for molding machine motion control apparatus and methodologies by which improved machine performance can be achieved in the presence of such conditions.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention, nor to delineate the scope of the invention. Its primary purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. The present invention is directed to control methods and apparatus finding application in association with various types of molding machines, such as injection molding machines, blow molding machines, and the like. A control law is provided, which estimates future machine states according to a proposed or hypothetical control output vector, a current machine state and a model of the machine, and recursively refines the control output vector using an error function and an adjustment rule in order to reduce the error between the estimated future machine states and desired future machine states.
The current machine state may be provided by an observer, using the model and current machine sensor signals, by which the number of required sensors can be reduced. Alternatively or in combination, the observer may provide the current machine state using a model with no sensor inputs, for instance, where a control output u(t) is known. One or more control outputs are provided to actuators associated with the machine according to the refined control output vector, where the actuators may be of any type, including but not limited to hydraulic and electric, linear and rotary actuators. The invention thereby achieves non-linear model predictive control with distinct advantages over conventional molding machine motion and/or pressure control apparatus and methodologies, via the employment of a non-linear model of the machine behavior and the iterative or recursive control output refinement. In this regard, the control apparatus and methodologies of the present invention can be advantageously employed so as to provide shorter rise times, reduced overshoot, improved disturbance rejection characteristics, and improved profile emulation as compared with conventional molding machine controls.
One aspect of the invention provides a system for controlling motion or pressure in a non-linear injection molding machine. The control system comprises a model and a control law, wherein the model comprises a plurality of state equations representative of behavior of the injection molding machine. The control law comprises a simulator component, which simulates estimated future states of the injection molding machine according to a proposed control output vector having control output values representative of control outputs at a current time and at future times. The simulator provides the estimated future states using a current state vector with state values representative of a current state of the injection molding machine and the machine model.
The control law further comprises an error function evaluator, which determines an error between the estimated future states and desired future states, such as a velocity profile, as well as an adjustment rule, which adjusts the proposed control output vector according to the error to provide a refined proposed control output vector. The control law refines the proposed control output vector using the adjustment rule, simulates refined estimated future states using the simulator, and determines a refined error between the refined estimated future states and the desired future states in recursive fashion until a termination condition occurs, such as when a maximum time has expired, a maximum number of iterations have occurred, or when the error or the rate of change thereof is less than a threshold. A control output is then provided to one or more actuators in the molding machine according to the most recently refined proposed control output vector.
According to another aspect of the invention, the control system may also comprise an observer receiving sensor inputs, which estimates the current state of the injection molding machine using the model and the sensor inputs. The model may comprise one or more differential equations, where the observer solves the differential equations using the proposed control output vector and the current state vector to provide the current state of the machine, including measured and/or unmeasured states. Similarly, the simulator operates to solve the differential equations to estimate future machine states. In one implementation, the differential equations of the model may be solved using a Runge-Kutta method. Alternatively, difference equation techniques can be employed to solve the differential equations of the model. Once the control output is thus determined for a given control cycle, the control law may save at least a portion of the most recently refined proposed control output vector. In a subsequent control cycle, the control law provides a first proposed control output vector according to a previously saved proposed control output vector from a prior control cycle. In this fashion, the invention provides for reducing the number of iterations required to achieve a given control output refinement.
The control law may further comprise a control output function and a control output parameter translator, wherein the adjustment rule is operative to correlate or generate one or more coefficients from the proposed control output vector according to the control output function. The adjustment rule then evaluates one or more partial differential equations for error with respect to the coefficients, and adjusts one or more of the coefficients so as to reduce the error, which may be done using a conjugate gradient technique in accordance with another aspect of the invention. The control output parameter translator then translates the adjusted coefficients according to the control output function to provide the refined proposed control output vector, which may then be used in a subsequent simulation.
In accordance with another aspect of the invention, a molding machine control system is provided, which comprises a model, an observer, and a control law, wherein the system controls motion and/or pressure in non-linear and/or linear molding machines. The observer receives sensor input values and estimates the current machine state using the model and the sensor values to provide a current state vector. The control law comprises a simulator, an error function evaluator, and an adjustment rule, wherein the simulator provides estimated future states according to a proposed control output vector using the current state vector and the model. The error function evaluator determines an error between the estimated future states and desired future states, and the adjustment rule adjusts the proposed control output vector according to the error to provide a refined proposed control output vector. The control law recursively refines the proposed control output vector until a termination condition occurs, and then provides a control output according to the most recently refined proposed control output vector.
Yet another aspect of the invention comprises methodologies for controlling motion and/or pressure in a non-linear injection molding machine. The methods comprise obtaining a current state vector, providing a proposed control output vector, and simulating a plurality of estimated future states according to the proposed control output vector using the current state vector and a model. The methods further comprise determining an error between the estimated future states and desired future states, and recursively refining the proposed control output vector according to the error. The refinement comprises simulating refined estimated future states according to the refined proposed control output vector using the current state vector and the model, and determining a refined error between the refined estimated future states and the desired future states until a termination condition occurs. Thereafter, a control output is provided to at least one actuator in the injection molding machine according to the most recently refined proposed control output vector. A portion of the final proposed control output vector may be saved for use in providing a first proposed control output vector in a subsequent control cycle.
The model may comprise one or more differential equations, wherein simulating the plurality of estimated future states comprises solving the differential equations using the proposed control output vector and the current state vector. The solution of the differential equations may be accomplished by numerical techniques, such as a Runge-Kutta method, and/or difference equation techniques. Obtaining the current state vector may comprise obtaining one or more sensor input values and estimating the current state of the injection molding machine using the model and the sensor input values, such as by computing state values representative of a current machine state using the model, and forming a current state vector using the computed state values. In addition, refining the proposed control output vector may comprise generating coefficients from the proposed control output vector according to a control output function, evaluating one or more partial differential equations for error with respect to the coefficients, and adjusting one or more of the coefficients so as to reduce the error. The coefficients may me adjusted, for example, using a conjugate gradient method. The adjusted coefficients are then translated according to the control output function to provide the refined proposed control output vector.
Still another aspect of the invention provides a method for controlling motion or pressure in an injection molding machine, comprising obtaining one or more sensor inputs, estimating a current machine state using the sensor inputs and a model, and providing a proposed control output vector having a plurality of control output values representative of control outputs at a current time and at future times. The method further comprises simulating estimated future states of the molding machine according to the proposed control output vector using the current state and the model, determining an error between the estimated future states and desired future states, and recursively refining the proposed control output vector. The refinement comprises simulating refined estimated future states of the injection molding machine according to the refined proposed control output vector using the current state and the model, and determining a refined error between the refined estimated future states and the desired future states until a termination condition occurs. A control output is then provided according to the most recently refined proposed control output vector.